Thermosiphon systems for electronic devices

ABSTRACT

A thermosiphon system includes a condenser and an evaporator fluidically coupled to the condenser by a condensate line. The evaporator includes a housing having an opening to the condensate line, a wick located in the housing, and a flow restrictor located in the housing configured to restrict flow of a working fluid from the condensate line onto a portion of the wick.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/093,609, filed Apr. 25, 2011, the entire content of which is herebyincorporated by reference.

TECHNICAL FIELD

This invention relates to thermosiphon systems to remove heat fromelectronic devices.

BACKGROUND

Computer users often focus on the speed of computer microprocessors(e.g., megahertz and gigahertz). Many forget that this speed often comeswith a cost—higher power consumption. This power consumption alsogenerates heat. That is because, by simple laws of physics, all thepower has to go somewhere, and that somewhere is, in the end, conversioninto heat. A pair of microprocessors mounted on a single motherboard candraw hundreds of watts or more of power. Multiply that figure by severalthousand (or tens of thousands) to account for the many computers in alarge data center, and one can readily appreciate the amount of heatthat can be generated. The effects of power consumed by the criticalload in the data center are often compounded when one incorporates allof the ancillary equipment required to support the critical load.

Many techniques may be used to cool electronic devices (e.g.,processors, memories, and other heat generating devices) that arelocated on a server rack tray. For instance, forced convection may becreated by providing a cooling airflow over the devices. Fans locatednear the devices, fans located in computer server rooms, and/or fanslocated in ductwork in fluid communication with the air surrounding theelectronic devices, may force the cooling airflow over the traycontaining the devices. In some instances, one or more components ordevices on a server tray may be located in a difficult-to-cool area ofthe tray; for example, an area where forced convection is notparticularly effective or not available.

The consequence of inadequate and/or insufficient cooling may be thefailure of one or more electronic devices on the tray due to atemperature of the device exceeding a maximum rated temperature. Whilecertain redundancies may be built into a computer data center, a serverrack, and even individual trays, the failure of devices due tooverheating can come at a great cost in terms of speed, efficiency, andexpense.

Thermosiphons are heat exchangers that operate using a fluid thatundergoes a phase change. A liquid form of the fluid is vaporized in anevaporator, and heat is carried by the vapor form of the fluid from theevaporator to a condenser. In the condenser, the vapor condenses, andthe liquid form of the fluid is then returned via gravity to theevaporator. Thus, the fluid circulates between the evaporator and thecondenser without need of a mechanical pump.

SUMMARY

As noted above, electronic devices, e.g., computer components, such asprocessors and memories, generate heat. A thermosiphon system can beused to remove heat from such an electronic device. Although somesystems have been proposed for removing heat from computer components,the limited space available in the server rack environment introduces anadditional challenge to thermosiphon system design. In addition, forcommercial applicability, the thermosiphon needs to operate with highefficiency.

Several approaches are described, which can be used individually or incombination in order to improve efficiency. The condenser can havemultiple vertical chambers, but lack a top header in order to fit withinthe limited vertical space of the server rack. The inner surfaces of thecondenser can include undulations that reduce thermal resistance acrossthe liquid film in the condenser. A flow restrictor in the evaporatorcan be used to form a thin layer of liquid in the evaporator over theregion where the evaporator contacts the electronic device, thusreducing thermal resistance of the evaporator.

In one aspect, a thermosiphon system includes a condenser and anevaporator fluidically coupled to the condenser by a condensate line.The evaporator includes a housing having an opening to the condensateline, a wick located in the housing, and a flow restrictor located inthe housing configured to restrict flow of a working fluid from thecondensate line onto a portion of the wick.

Implementations can include one or more of the following. The housingmay have a bottom interior surface, the wick may be positioned on thebottom interior surface, and the flow restrictor may include afluid-impermeable barrier on the bottom interior surface between thewick and the opening. The barrier may have a plurality of aperturestherethrough to permit the working fluid to flow to the wick. Theplurality of apertures through the barrier may be positioned adjacentthe bottom interior surface. The bottom interior surface may be a planarsurface. The fluid-impermeable barrier may dam the working fluid so thatthe working fluid pools on a side of the barrier closer to the opening.The flow restrictor may be configured such that a depth of the workingfluid is greater over a region of the housing between the barrier andthe opening than over the portion of the wick. The fluid-impermeablebarrier may surround the portion of the wick. The housing may include atop interior surface and there may be a gap between the barrier and theinterior top surface of the housing. A vapor line may fluidically couplethe evaporator to the condenser, an opening in the housing to the vaporline may be located in an interior top surface of the housing, and theopening in the housing to the condensate line may be located in aninterior side surface of the housing. The condensate line may include acombined condensate and vapor transfer line, and fluidical couplingbetween the evaporator and the condenser may consist of the combinedcondensate and vapor transfer line.

In another aspect, a thermosiphon system includes an evaporator, acondenser comprising a plurality of parallel vertically-extendingchambers, the chambers having closed off top ends, and a condensate linefluidically coupling the condenser to the evaporator.

Implementations can include one or more of the following features. Thecondenser may include a bottom header and a plurality of condensatetubes projecting upwardly from the bottom header, and the plurality ofparallel vertically-extending chambers may be located within theplurality of condensate tubes. The condensate line may fluidicallycouples the bottom header of the condenser to the evaporator. Thecondenser does not include a top header. The condenser may include aplurality of heat conducting fins projecting outwardly from thecondensate tubes. The condensate tubes may extend perpendicular to thebottom header and the heat conducting fins may extend parallel to thebottom header. The condenser may include a body having a cavity formedtherein and a plurality of walls in the cavity that divide the cavityinto the plurality of parallel vertically-extending chambers. Theplurality of vertically-extending chambers may extend laterally from acentral channel. A first set of the plurality of vertically-extendingchambers may extend laterally from a first side of the central channel,and a second set of the plurality of vertically-extending chambers mayextend laterally from an opposite second side of the central channel. Aplurality of heat conducting fins may project outwardly from the body.The plurality of heat conducting fins may project vertically from thebody. The plurality of heat conducting fins may be orientedperpendicular to the vertically-extending chambers.

In another aspect, a thermosiphon system includes an evaporator, acondenser comprising an interior volume bounded by a substantiallyvertical interior surface, a condensate line fluidically coupling thecondenser to the evaporator. The interior surface includes undulationsprojecting inwardly into the interior volume on a second axisperpendicular to a vertical first axis with peaks of the undulationsspaced apart along a third axis perpendicular to the first axis and thesecond axis.

Implementations can include one or more of the following features. Theinterior volume may have a length along the third axis and a width alongthe second axis and the length is greater than the width. The peaks ofthe undulations may be spaced regularly along the third axis. Theundulations may have a pitch along the third axis between 0.1 and 1 mm.The undulations may have an amplitude along the second axis between 0.1and 1 mm. The undulations may have pitch along the third axis and anamplitude along the second axis, and a ratio of the pitch to theamplitude is between about 1:1 to 2:1. The undulations may be sinusoidalwaves. The undulations may be a plurality of curved segments in whichdK/dS equal to a constant value, where K is an inverse of the radius ofcurvature of the undulation and S is a distance along the curvedsegments.

In another aspect, a thermosiphon system includes an evaporator, acondenser comprising a plurality of parallel chambers connected to acommon channel, and a condensate line fluidically coupling the commonchannel of the condenser to the evaporator. The condenser is located ata height above the evaporator with a liquid phase of a working fluidthat fills a bottom portion of an interior volume of the condensateline, a top surface of the liquid phase has a non-zero angle relative tohorizontal from the condenser to the evaporator, and a vapor phase ofthe working fluid can pass through a top portion of the interior volumeof the condensate line, the top portion extending from the condenser tothe evaporator.

In another aspect, a server rack sub-assembly includes a tray configuredfor slidable insertion into a server rack, a motherboard mounted on thetray and laying in a plane, an underside of the motherboard separatedfrom the tray by a gap, a heat-generating electronic device mounted on atop side of the motherboard, and a thermosiphon system. The thermosiphonsystem includes an evaporator supported on the heat generatingelectronic device and a condenser supported on the tray and fluidicallycoupled to the evaporator. The evaporator has a bottom surfacepositioned adjacent and thermally conductively coupled to theheat-generating electronic device. The condenser includes a condensatecollector positioned above the plane of the motherboard and a pluralityof heat conducting fins extending downwardly from the condensatecollector below the plane of the motherboard. A fan is mounted on thetray and oriented to generate an airflow over the motherboard andbetween the fins of the evaporator.

Implementations can include one or more of the following. A secondheat-generating electronic device may be mounted on the motherboard, thethermosiphon system may include a second evaporator, and the secondevaporator may include a second bottom surface positioned adjacent andthermally conductively coupled to the second heat-generating electronicdevice. The tray may be configured for insertion into a 13 inch or 19inch server rack. A total height from a bottom of the tray to a top ofthe heat conducting fins may be at most 6 inches.

One or more of the following advantages may be realized. Thethermosiphon system can fit within the limited horizontal and verticalspace of the server rack. A thin layer of liquid can be maintained inthe evaporator over the region where the evaporator contacts theelectronic device, thus reducing thermal resistance of the evaporator toabsorption of heat from the electronic device. In addition, thelikelihood of flooding of this region can be reduced, thus reducing thelikelihood of failure of the thermosiphon system due to increasedthermal resistance. Undulations in the internal surfaces of thecondenser can result in a thin layer of liquid, thus reducing thethermal resistance of the condenser, and thereby improving efficiency ofradiation of heat out of the thermosiphon system.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects,features, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a side view of a server rack and a server-racksub-assembly configured to mount within the rack.

FIGS. 2 and 3 illustrate a side view and a top view of a server racksub-assembly.

FIG. 4 illustrates a perspective view of a server rack sub-assembly (butomits the printed circuit board and heat generating elements to providea view of more of the frame).

FIGS. 5 and 6 illustrate a top view and a side view, cross-sectional, ofan evaporator from the thermosiphon system.

FIG. 7 illustrates a perspective view, partially cut away, of anevaporator from the thermosiphon system.

FIGS. 8 and 9 illustrate side views, cross-sectional, of a condenserfrom the thermosiphon system.

FIGS. 10 and 11 illustrate top views, cross-sectional, of thethermosiphon system of FIGS. 8 and 9.

FIG. 12 illustrates a perspective view, cut away, of a condenser fromthe thermosiphon system.

FIGS. 13 and 14 illustrate a top view and a side view, cross-sectional,of another implementation of a condenser.

FIG. 15 illustrates a perspective view, cut away, of the othercondenser.

FIG. 16 is an expanded top view, cross-sectional, of a chamber in thecondenser.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This document discusses a thermosiphon system that can be implemented toremove heat from an electronic device, e.g., a component of computingequipment, such as a processor or memory. The evaporator of thethermosiphon system contacts the electronic device so that theelectronic device experiences a conductive heat transfer effect. Thus,the thermosiphon system can act as a heat sink for the electronicdevice, reducing the likelihood of overheating and subsequent failure ofthe electronic device.

In particular, the thermosiphon system can be mounted on or integratedwith a server rack sub-assembly for insertion into a server rack. Theserver rack sub-assembly can contain or support a number ofheat-generating electronic devices, and the evaporator of thethermosiphon system can contact one or more of the electronic devices.In addition, the thermosiphon system can be mounted on a circuit cardassembly, a daughter card, and/or other boards that carryheat-generating electronic devices.

FIG. 1 illustrates an example system 100 that includes a server rack105, e.g., a 13 inch or 19 inch server rack, and multiple server racksub-assemblies 110 mounted within the rack 105. Although a single serverrack 105 is illustrated, server rack 105 may be one of a number ofserver racks within the system 100, which may include a server farm or aco-location facility that contains various rack mounted computersystems. Also, although multiple server rack sub-assemblies 110 areillustrated as mounted within the rack 105, there might be only a singleserver rack sub-assembly. Generally, the server rack 105 definesmultiple slots 107 that are arranged in an orderly and repeating fashionwithin the server rack 105, and each slot 107 is a space in the rackinto which a corresponding server rack sub-assembly 110 can be placedand removed. For example, the server rack sub-assembly can be supportedon rails 112 that project from opposite sides of the rack 105, and whichcan define the position of the slots 107. The slots, and the server racksub-assemblies 110, can be oriented with the illustrated horizontalarrangement (with respect to gravity). Alternatively, the slots 107, andthe server rack sub-assemblies 110, can be oriented vertically (withrespect to gravity), although this would require some reconfiguration ofthe evaporator and condenser structures described below. Where the slotsare oriented horizontally, they may be stacked vertically in the rack105, and where the slots are oriented vertically, they may be stackedhorizontally in the rack 105.

Server rack 105, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 105 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 105 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 110 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 110 may be a “tray” ortray assembly that can be slidably inserted into the server rack 105.The term “tray” is not limited to any particular arrangement, butinstead applies to motherboard or other relatively flat structuresappurtenant to a motherboard for supporting the motherboard in positionin a rack structure. In some implementations, the server racksub-assembly 110 may be a server chassis, or server container (e.g.,server box). In some implementations, the server rack sub-assembly 110may be a hard drive cage.

Referring to FIGS. 2-4, the server rack sub-assembly 110 includes aframe or cage 120, a printed circuit board 122, e.g., motherboard,supported on the frame 120, one or more heat-generating electronicdevices 124, e.g., a processor or memory, mounted on the printed circuitboard 122, and a thermosiphon system 130. One or more fans 126 can alsobe mounted on the frame 120.

The frame 120 can include or simply be a flat structure onto which themotherboard 122 can be placed and mounted, so that the frame 120 can begrasped by technicians for moving the motherboard into place and holdingit in position within the rack 105. For example, the server racksub-assembly 110 may be mounted horizontally in the server rack 105 suchas by sliding the frame 120 into the slot 107 and over a pair of railsin the rack 105 on opposed sides of the server rack sub-assembly110—much like sliding a lunch tray into a cafeteria rack. Although FIGS.2 and 3 illustrate the frame 120 extending below the motherboard 122,the frame can have other forms (e.g., by implementing it as a peripheralframe around the motherboard) or may be eliminated so that themotherboard itself is located in, e.g., slidably engages, the rack 105.In addition, although FIG. 2 illustrates the frame 120 as a flat plate,the frame 120 can include one or more side walls 121 (see FIG. 4) thatproject upwardly from the edges of the flat plate, and the flat platecould be the floor of a closed-top or open-top box or cage.

The illustrated server rack sub-assembly 110 includes a printed circuitboard 122, e.g., a motherboard, on which a variety of components aremounted, including heat-generating electronic devices 124. Although onemotherboard 122 is illustrated as mounted on the frame 120, multiplemotherboards may be mounted on the frame 120, depending on the needs ofthe particular application. In some implementations, the one or morefans 126 can be placed on the frame 120 so that air enters at the frontedge (at the left hand side in FIG. 3) of the server rack sub-assembly110, closer to the front of the rack 105 when the sub-assembly 110 isinstalled in the rack 105, flows (see arrow A in FIG. 4) over themotherboard and over some of the heat generating components on themotherboard 122, and is exhausted from the server rack assembly 110 atthe back edge (at the right hand side in FIG. 3), closer to the back ofthe rack 105 when the sub-assembly 110 is installed in the rack 105. Theone or more fans 126 can be secured to the frame 120 by brackets 127.Thus, the fans 126 can pull air from within the frame 120 area and pushthe air after it has been warmed out the rack 105. An underside of themotherboard 122 can be separated from the frame 120 by a gap.

The thermosiphon system 130 includes an evaporator 132, a condenser 134,and condensate/vapor lines 136 connecting the evaporator 132 to thecondenser 134. The evaporator contacts the electronic device 124 so thatheat is drawn by conductive heat transfer from the electronic device 124to the evaporator 132. In particular, the bottom of the evaporator 132contacts the top of the electronic device 124. In operation, heat fromthe electronic device 124 causes a working fluid in the evaporator 132to evaporate. The vapor then passes through condensate/vapor lines 136to the condenser 134. Heat is radiated away from the condenser 134,e.g., into air blown or drawn by the one or more fans 126 that passesacross the condenser 134, causing the working fluid to condense. Asshown in FIG. 2, the condenser 134 can be located on an opposite side ofone or more of the one or more fans 126 from the evaporator 132.Alternatively or in addition, the condenser 134 can be located on a sameside of one or more of the one or more fans 126 as the evaporator 132.

As shown in FIG. 2, the condensate/vapor line 136 is at a slight(non-zero) angle so that gravity causes the condensed working fluid toflow back through the condensate/vapor lines 136 to the evaporator 132.Thus, in some implementations, at least a portion of thecondensate/vapor lines 136 is not parallel to the main surface of theframe 120. For example, the condenser-side end of the condensate/vaporline 136 can be about 1-5 mm, e.g., 2 mm, above the evaporator-side endof the condensate/vapor line 136. However, it is also possible for thecondensate/vapor line 136 to be horizontal tube, or even at a slightlynegative angle (although the positive angle provides an advantage ofgravity improving flow of the liquid from the condenser to theevaporator). Because there can be multiple heat generating electronicdevices on a single motherboard, there can be multiple evaporators onthe motherboard, where each evaporator corresponds to a singleelectronic device. As shown in FIG. 2, there is a first evaporator 132and a second evaporator 132 as well as a first electronic device 124 anda second electronic device 124. The condensate/vapor line 136 connectingthe first evaporator to the second evaporator can be level.

During operation, the top surface of the liquid inside the condenserwill be above the top surface liquid height in the evaporator, e.g., by1 to 10 mm. It can be easier to achieve this with a condensate/vaporline 136 that is at a slight (positive non-zero) angle, but properselection of the thermal and mechanical properties of the working fluidin view of the expected heat transport requirements for the thermosiphonsystem may still achieve this for a condensate/vapor line 136 that ishorizontal or at a slightly negative angle. During operation, the liquidphase of a working fluid can fill a bottom portion of an interior volumeof the condensate/vapor line 136, with the bottom portion extending fromthe condenser to the evaporator, and a vapor phase of the working fluidcan pass through a top portion of the interior volume of thecondensate/vapor line 136, with the top portion extending from thecondenser to the evaporator.

In some implementations, the condenser 134 can be located at a heightabove the evaporator 132 such that a liquid phase of the working fluidfills a bottom portion of an interior volume of the condensate/vaporline 136 and such that during operation a top surface of the liquidphase has at a non-zero angle relative to horizontal from the condenser132 to the evaporator 134, and a vapor phase of the working fluid canpass through a top portion of the interior volume of thecondensate/vapor line 136, the top portion extending from the condenser132 to the evaporator 134.

FIGS. 2-4 illustrate a thermosiphon system 130 with multiple evaporators132; each evaporator 132 can contact a different electronic device 124,or multiple evaporators 132 could contact the same electronic device,e.g., if the electronic device is particularly large or has multipleheat generating regions. As shown in FIGS. 2-4, the multiple evaporators132 can be connected by the condensate/vapor lines 136 to the condenser134 in series, i.e., a first condensate/vapor line connects thecondenser to a first evaporator, and a second condensate/vapor lineconnects the first evaporator to a second evaporator. Alternatively,some or all of the multiple evaporators 132 can be connected by thecondensate/vapor lines 136 to the condenser 134 in parallel, i.e., afirst condensate/vapor line connects the condenser to a firstevaporator, and a second condensate/vapor line connects the condenser134 to a second evaporator. Advantages of a serial implementation isfewer tubes, whereas an advantage of parallel tubes is that the tubediameters can be smaller.

FIGS. 2-4 illustrate a thermosiphon system 130 in which a common line isused for both the condensate flow from the condenser 134 to theevaporator 132 and for vapor flow from the evaporator 132 to thecondenser 134. Thus, in this implementation the fluidical couplingbetween the evaporator 132 and the condenser 134 consists of thecombined condensate and vapor transfer line. In some implementations,there can be separate lines for the vapor and the condensate. However, apotential advantage of the combined condensate and vapor transfer lineis that the line can be connected to a side of the condenser, reducingthe vertical height of the system relative to a system with a separateline for the vapor, since the vapor line is typically coupled to or nearthe top of the evaporator. The condensate/vapor line 136 can be a tubeor pipe, e.g., of copper or aluminum.

FIGS. 5-7 illustrate an evaporator 132 that includes a housing thatencloses a chamber 146, a wick 142, and a flow restrictor 150. Thehousing can include a base 140 and a case 144 that is secured to thebase 140. The housing has an opening to connect the chamber 146 to thecondensate line 136 (or an opening for each line 136 if there aremultiple lines 136), but the chamber 146 can otherwise be sealed andfluid-tight.

The flow restrictor 150 is configured to restrict flow of a workingfluid from the condensate line 136 onto a portion of the wick 142. Inoperation, a working fluid 160, in liquid form, flows from thecondensate line 136 into the chamber 146 and pools before the flowrestrictor 150 (see FIG. 6). The flow restrictor 150 permits a smallportion of the working fluid to pass, creating a thin layer 162 of theworking fluid 160 on an active area on the bottom interior surface ofthe housing, e.g., on top of the base 140. By creating a thin layer 162of the working fluid, the thermal resistance of the evaporator iseffectively reduced (because the working fluid can evaporate morereadily from a thin layer, permitting greater heat transfer).

The base 140 is formed of a thermally conductive material, e.g., copper.The housing, e.g., the bottom of the base 140, can directly contact theelectronic device 124, e.g., the top surface of the electronic device124. Alternatively, the housing, e.g., the bottom of the base 140, canbe connected to the electronic device 124, e.g., the top surface of theelectronic device 124, by a thermally conductive interface material 141,e.g., a thermally conductive pad or layer, e.g., a thermally conductivegrease or adhesive.

The wick 142 can be formed on the bottom interior surface of thehousing, e.g., on the top surface of the base 140. The wick 142 is athermally conductive area that transfers heat from the base 140 to theworking fluid 160. In addition, the wick 142 can be configured to drawthe working fluid 160 by capillary action. In some implementations, thewick 142 can be a porous or roughened region of the top surface of thebase 140. For example, the wick 142 can be formed from copper particlesthat are bonded to the top surface of the base 140. For example, copperparticles with a mean diameter of 25 microns can deposited to a depth ofabout 0.25 mm on the top surface of the base 140 and then sintered.Other possibilities for the wick 142 include a porous material that sitson the base 140, microgrooving on the base 140, or a screen wick. Apartfrom the roughness of the wick 142, the bottom interior surface of thehousing can be a planar surface.

The flow restrictor 150 can be a barrier 150 a of fluid-impermeablematerial on the bottom interior surface of the housing between the wick142 and the opening to the condensate line 136. The barrier 150 apartitions the bottom interior surface of the housing into a firstportion 152 into which the liquid working fluid can flow easily, and asecond portion 154 into which flow of the working fluid is restricted.That is, the working fluid must pass under or through the barrier inorder to flow from the first portion 152 to the second portion 154. Thefirst portion 152 can be adjacent the opening to the condensate line136. The second portion 154 can be positioned directly over theelectronic device 124. Thus, the active area of the bottom interiorsurface of the housing that receives the most heat from the electronicdevice 124 can be the region in which the thin layer 162 of the workingfluid is created.

The barrier 150 a can surround part or all of the wick 142, so that thesecond portion 154 can partially or entirely overlie the wick 142 (thebarrier 150 a can still be considered to “surround” a portion of thewick 142 when it rests on the wick 142). Optionally, some portion of thewick 142 can extend past the barrier 150 a into the second portion 154.In the implementation illustrated in FIGS. 5-7, the barrier 150 a andthe first portion 152 entirely surround the second portion 154, such asthe horizontal plane. However, in some implementations the secondportion 154 could abut the housing, e.g., abut the case 144, so that awall of the housing forms part of the perimeter of the second portion154, with the barrier 150 a providing a remainder of the perimeter.

The flow restrictor 150, e.g., the barrier 150 a, can have a pluralityof apertures 156 therethrough to permit the liquid working fluid to flowinto the second portion 154. The plurality of apertures can bepositioned adjacent the bottom interior surface of the housing, e.g.,adjacent the top surface of the base 140. The apertures are sized basedon the thermal properties of the working fluid and the expected heattransfer of the thermosiphon system such that a small portion of theworking fluid passes through the barrier 150 a, creating a layer 162 ofthe working fluid 160 on the active area. In addition or in thealternative, where a portion of the barrier 150 a rests on the wick 142,working fluid could be pulled below the barrier 150 a through the wick142 itself.

The barrier 150 a dams the working fluid 160 so that a portion of theworking fluid pools on a side of the barrier 150 a closer to theopening, e.g., over the first portion 152 of the bottom interiorsurface. In short, the flow restrictor is configured such that a depthof the working fluid is greater over a region of the housing between thebarrier and the opening, e.g., over the first portion 152, than over theportion of the wick, e.g., than over the second portion 154.

The housing includes a top interior surface, e.g., provided by the case144. There can be a gap between the barrier 150 a and the top interiorsurface. The opening from the chamber 144 to the condensate line 136 canbe located in an interior side surface of the housing. For example, theopening from the chamber 144 to the condensate line 136 can bepositioned adjacent the bottom interior surface of the housing, e.g.,adjacent the top surface of the base 140. If a separate vapor line ispresent, then an opening in the housing to the vapor line can be locatedin the top interior surface of the housing, e.g, in the ceiling of thecase 144, or in a side of the case at a position vertically higher thanthe opening to the condensate line 136. The case 144 can be atransparent material to permit observation of the interior of theevaporator 132.

Although the housing composed of the base 142 and case 144 illustratedby FIGS. 5-7 is a rectangular solid, this is not required, and thehousing could be another right solid, e.g., cylindrical, or some othershape. Similarly, although the first and second portions 152, 154 of thebottom interior surface of the housing are rectangular, other simplepolygons, e.g., convex polygons, or non-self-intersecting curved shapes,e.g., circles or ellipses, are possible, and the first and secondportions need not be geometrically similar.

The condenser 132 includes a plurality of chambers, and a plurality ofheat conducting fins. The chambers can be parallel andvertically-extending. The top ends of the chambers can be closed off,i.e., there is no top header that interconnects the top ends of thechambers.

FIGS. 8-12 illustrate a first implementation of the condenser 134 thathas a body 170 having cavity 174 formed therein, and a plurality ofwalls 172 in the cavity that divide the cavity 174 into a plurality ofparallel vertically-extending chambers 174 a. The chambers 174 a can beparallel and vertically-extending. The top ends of the chambers 174 acan be closed off, i.e., there is no top header that interconnects thetop ends of the chambers 174 a. The walls 172 act as a condensationsurface and to conduct heat from the vapor, through the body to thefins.

The cavity 174 also includes a central channel 176 with an opening tothe exterior of the body 170 which is coupled to the condensate line136. The vertically-extending chambers 174 a can extend laterally fromthe central channel 176, and the chambers 174 a can extend parallel tothe long axis of the body 170 (i.e., the body has a length greater thanits width, and the long axis is along the length). The central channel176 can extend laterally perpendicular to the long axis. When thecondenser 134 is installed on the frame, the central channel 176 can runfrom the front toward the rear of the body 170. A first set of thevertically-extending chambers 174 can extend laterally from a first sideof the central channel 176, and a second set of the plurality ofvertically-extending chambers 174 can extend laterally from an oppositesecond side of the central channel 176. The body 170 can be a generallyrectangular solid, although other shapes are possible.

FIGS. 8-12 illustrate a first implementation of the condenser 134 thathas a plurality of heat conducting fins 180 that project outwardly fromthe body 170. For example, the fins 180 can project vertically from thebody 170. The fins 170 can be generally flat, narrow sheets. The fins180 can project in parallel to each other from the body 170, and can bespaced apart with a regular pitch along a direction normal to their flatprimary surfaces. In some implementations, the fins 180 include at leasta first plurality of fins 180 a that project upwardly from the topsurface of the body 170. In some implementations, the fins 180 alsoinclude a second plurality of fins 180 b that project downwardly fromthe bottom surface of the body 170.

When the condenser 134 is installed on the frame, the fins 180 can beoriented with their length extending parallel or generally parallel tothe direction of air flow generated by the fans, e.g., with their lengthrunning from the front toward the rear of the of the body 170. The fins180 can be oriented with their long axis perpendicular to, or at aslight angle to, the long-axis of the chambers 174 a and/or the body170.

Returning to FIG. 2, the condenser 134 can rest on the frame 120, andthe fins 180 b that project downwardly from the bottom surface of thebody 170 can project below the plane of the motherboard 122. This canimprove the available surface area for the fins to improve radiatingefficiency of the condenser 134. This can also assist in limiting thevertical height of the condenser 134 so that the thermosiphon system 130is compatible with the limited height available in the server rackenvironment. For example, a total height from a bottom of the tray to atop of the heat conducting fins can be at most 6 inches, e.g., at most 4inches.

FIGS. 13-15, illustrate a second implementation of the condenser 134that also has a plurality of heat conducting fins 180 that projectoutwardly from the body 170. However, in this implementation, thevertically-extending chambers 174 a extend vertically from the centralchannel 176. In particular, the body can include a bottom header 190which contains the central channel 176, and plurality of tubes 192 thatproject vertically from the bottom header 190 and contain thevertically-extending chambers 174 a. The condensate line 136 isfluidically coupled to the bottom header 190 of the condenser 134.

Each chamber 174 a can be formed by its own, and the walls 172 that formthe boundaries of vertically extending chamber 174 a can be walls of thetubes 192. The chambers 174 a can extend perpendicular to the long axisof the body 170. Although the vertically extending chambers 174 a areconnected to a bottom header 190, the top ends of the chambers 174 a canbe closed off, i.e., the condenser 134 does not include a top header.

The fins 180 can project horizontally from the body 170, e.g.,horizontally from the tubes 192. The fins 180 can extend parallel to thelong axis of the bottom header 190. The fins 180 can be generally flat,narrow sheets. The fins 180 can project in parallel to each other fromthe body 170, and can be spaced apart, e.g., vertically spaced apart,with a regular pitch along a direction normal to their flat primarysurfaces.

When the condenser 134 is installed on the frame, the fins 180 can beoriented with their length extending parallel or generally parallel tothe direction of air flow generated by the fans, e.g., with their lengthrunning from the front toward the rear of the of the body 170. The fins180 can be oriented with their long axis parallel to the long-axis ofthe chambers 174 a. The chambers 174 a canshorter than they are long.

In either implementation of the condenser, both the body 170 of thecondenser 134 and the fins 180 can be formed of a material with a goodterminal conductivity, comparable or better than aluminum, e.g., of atleast 200 W/mK. A nickel plating can be used to solder the fins 180 tothe body 170, or the fins 180 can be brazed to the body 170.

Referring to FIGS. 11, 13 and 16, at least some interior surfaces of thecondenser, e.g., surfaces that bound the cavity 174, can optionally betexturized. The texturization can apply to either implementation of thecondenser. The cavity 174 provides an interior volume bounded by asubstantially vertical interior surface, e.g., a surface of one of thewalls 172. The texturization of the interior surface can includeundulations projecting inwardly into the interior volume. Theundulations can be uniform along a vertical first axis, and can projectinto the interior volume along a second axis perpendicular to thevertical first axis. Peaks of the undulations can be spaced apart, e.g.,with a regular pitch, along a third axis perpendicular to the first axisand the second axis. The third axis can be parallel to the long axis ofthe body 170 and/or the chamber 174 a. Each chamber 174 a can have alength along the third axis and a width along the second axis with thelength being greater than the width. The undulations can be smooth,e.g., no discontinuities in the surface along the second axis.

The undulations can have a pitch along the third axis between 0.1 and 1mm and can have an amplitude along the second axis between 0.1 and 1 mm.In some implementations, a ratio of the pitch to the amplitude isbetween about 1:1 to 2:1. In some implementations, the undulations canform a sinusoidal wave. In some implementations, the undulations areformed by a plurality of curved segments in which dK/dS is equal to aconstant value, where K is an inverse of the radius of curvature of theundulation and S is a distance along a curved segment. Other shapes forthe undulations are possible. These undulations can cause thinning ofthe film of condensed working fluid that forms on the vertical interiorsurface, thereby reducing the thermal resistance of the condenser.

The working fluid can be a dielectric, non-flammable fluid with lowtoxicity, although but hydrocarbons such as methanol, ethanol or acetonecan also be suitable. The composition of the working fluid and internalpressure of the thermosiphon system can be selected to provide a boilingpoint of the working fluid in the evaporator at about the desiredoperating temperature for the electronic devices, e.g., around 30-100°C., e.g., 45-55° C. Examples of the working fluid include Vextral XFsold by DuPont, Flourinet Electronic Liquid FC-72, sold by 3M, and Novec7100, sold by 3M.

The entire interior of the thermosiphon system 130, including theinterior of the evaporator 132, condenser 134 and vapor/condensate line136, are vacuum filled and sealed. Initial vacuum can be pulled toachieve an internal absolute pressure below 0.05 millibar (5 Pa) toremove air from the thermosiphon system 130, and then the working fluidcan be introduced into thermosiphon system 130.

Although a server rack sub-assembly has been described above, thethermosiphon system could be used with heat-generating electronicdevices mounted on a motherboard that is not part of a server racksub-assembly, e.g., on a motherboard in a desktop computer, or could beused with heat-generating electronic devices that are not mounted on amotherboard.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A server rack sub-assembly, comprising: a trayconfigured for slidable insertion into a server rack; a motherboardmounted on the tray and a heat-generating device mounted on themotherboard; and a thermosiphon system including: an evaporatorsupported on the heating generating device, a condenser supported on thetray, the condenser comprising a body, the body including: a cavity, anda plurality of walls that divide the cavity into a central channel thatextends laterally through the body along a first axis and a plurality ofparallel chambers extending laterally away from the central channelalong a second axis perpendicular to the first axis, each chamber havinga length along the second axis greater than a width along the first axisand a height along a vertical axis greater than the width along thefirst axis, and a condensate line fluidically coupling the centralchannel of the condenser to the evaporator, the condensate lineextending laterally from the condenser body and the central channel andperpendicular to the chambers.
 2. The server rack sub-assembly of claim1, wherein the chambers have closed off top ends.
 3. The server racksub-assembly of claim 1, wherein a first set of the plurality ofchambers extend laterally from a first side of the central channel and asecond set of the plurality of chambers extend laterally from anopposite second side of the central channel.
 4. The server racksub-assembly of claim 1, further comprising a plurality of heatconducting fins projecting outwardly from the body.
 5. The server racksub-assembly of claim 4, wherein the plurality of heat conducting finsproject vertically from the body.
 6. The server rack sub-assembly ofclaim 5, wherein the plurality of heat conducting fins extend along thefirst axis.
 7. The server rack sub-assembly of claim 1, wherein thecondensate line extends perpendicular to the second axis.
 8. The serverrack sub-assembly of claim 7, wherein the condensate line is coupled toan end of the central channel.
 9. The server rack sub-assembly of claim1, wherein the plurality of walls comprises undulations projectinginwardly into the chambers.
 10. The server rack sub-assembly of claim 9,wherein the undulations project inwardly on the first axis with peaks ofthe undulations spaced apart along the second axis.
 11. The server racksub-assembly of claim 1, further comprising a fan mounted on the trayand oriented to generate an airflow over the motherboard and between aplurality of heat conducting fins that projecting outwardly from thecondenser.
 12. The server rack sub-assembly of claim 1, wherein theheat-generating device comprises a computer chip.
 13. The server racksub-assembly of claim 1, wherein the central channel has a same heightas the parallel chambers.
 14. The server rack sub-assembly of claim 1,wherein the central channel comprises two parallel channel wallsextending from a bottom surface to a top surface of the condenser body,the channel walls having a plurality of openings from the bottom surfaceto the top surface, the openings extending laterally off the centralchannel to a wall of the condenser body to create the chambers.
 15. Aserver rack sub-assembly, comprising: a tray configured for slidableinsertion into a server rack in a horizontal orientation; a motherboardmounted on the tray and a heat-generating device mounted on themotherboard; and a thermosiphon system including: an evaporatorsupported on the heating generating device, a condenser supported on thetray at a height above the evaporator, the condenser comprising a bodyhaving a plurality of parallel chambers connected to a common channel,and a single condensate line fluidically coupling the condenser to theevaporator, the condensate line extending from the common channel of thecondenser body, the condensate line including a first horizontal portionextending from the evaporator and a second portion tilted at an anglegreater than zero relative to horizontal that fluidically couples thefirst portion to the condenser, wherein an interior of the singlecondensate line includes a lower section coupled to the condenser at aposition to receive liquid working fluid from the condenser such thatthe first horizontal portion and the second portion tilted at the anglegreater than zero carry liquid working fluid from the condenser to theevaporator primarily by gravity and an upper section coupled to thecondenser at a position to return gaseous working fluid to thecondenser.
 16. The server rack sub-assembly of claim 15, furthercomprising a second heat-generating device mounted on the motherboard,wherein the thermosiphon system includes a second evaporator supportedon the second heat-generating device.
 17. The server rack sub-assemblyof claim 16, comprising a horizontal second condensate line fluidicallycoupling the second evaporator to the evaporator.
 18. The server racksub-assembly of claim 15, wherein the tray is configured for insertioninto a 13 inch or 19 inch server rack.
 19. The server rack sub-assemblyof claim 15, wherein the condenser includes fins projecting from a topof the body, and wherein a total height from a bottom of the tray to atop of the heat conducting fins is at most 6 inches.
 20. The server racksub-assembly of claim 15, wherein the chambers have a length greaterthan a width and a width greater than a depth, and the length of thechambers is parallel to the tray and perpendicular to the condensateline, and the depth of the chambers is parallel to the tray and thecondensate line.
 21. The server rack sub-assembly of claim 15,comprising a fan to blow air across the condenser, the fan positionedbetween the motherboard and the condenser.